(a) Field of the Invention
The present invention relates to a semiconductor polishing technique, and more particularly, to a semiconductor polishing technique by which a semiconductor wafer is brought into contact with a polishing pad.
(b) Description of the Related Art
Conventionally, a material wafer for manufacturing a semiconductor device is formed by growing a single-crystal semiconductor ingot of silicon or the like by the Czochralski method (CZ method) or the floating zone method (FZ method), abrading and shaping the outer periphery of the semiconductor ingot with the use of a cylindrical grinding machine or the like, and slicing the semiconductor ingot with a wire saw in a slicing process. After that, chamfering is performed on the wafer peripheral portions, and flattening and etching are also performed in a wrapping process. Primary polishing (rough polishing) and secondary polishing (finishing) are then performed to form a mirror wafer.
Circuits are formed on the surface of the mirror wafer obtained through the above-mentioned procedures, so as to form a semiconductor device. However, if the surface flatness of the wafer manufactured through the above-described procedures is low, part of the lens used in the exposure in a photolithography process for forming circuits does not come into focus, and it becomes difficult to form a minute circuit pattern.
Therefore, a very high degree of flatness is required in today's high-precision device manufacture. In the manufacture of wafers having a high degree of flatness, the wafer surface polishing is essential. As a polishing device for performing wafer surface polishing, a batch-type one-side polishing device has been widely known.
FIGS. 10-1 and 10-2 show an example of the batch-type one-side polishing device (see Japanese Patent Application Laid-Open (JP-A) No. 2006-117775). FIG. 10-1 is a vertical cross-sectional view of the batch-type one-side polishing device. FIG. 10-2 is an enlarged cross-sectional view of essential components of the batch-type one-side polishing device. The batch-type one-side polishing device is a device that polishes only one side of each of the wafers in one polishing operation, and is capable of polishing more than one wafer at the same time.
In FIGS. 10-1 and 10-2, the batch-type one-side polishing device 100 includes: a disk-like fixed platen 102 that is capable of rotating in a predetermined direction (counterclockwise when seen from above, for example); a polishing pad 104 that is formed with unwoven fabric or urethane foam bonded onto the surface of the fixed platen 102; polishing heads 108 that are placed above the polishing pad 104 and rotate about supporting axes 106; carrier plates 110 that are placed under the lower faces of the polishing heads 108; templates 112 that are fixed under the lower faces of the carrier plates 110 and hold wafers W with wafer positioning holes 112a; and a slurry tube 114 that supplies slurry to the surface of the polishing pad 104.
Each of the carrier plates 110 is the carrier for holding wafers, and is formed with porous resin such as polyurethane-resin porous solid. Each of the templates 112 is formed with glass epoxy resin, a polycarbonate sheet, a polyester sheet, or the like. Each of the templates 112 has five wafer positioning holes 112a for holding five wafers W. As shown in FIG. 10-2, the diameter of each of the wafer positioning holes 112a is larger than the wafer diameter. When the polishing heads 108 rotate, the wafers W freely rotate inside the wafer positioning holes 112a. 
In the batch-type one-side polishing device 100 shown in FIGS. 10-1 and 10-2, the templates 112 are provided to the carrier plates 110, so as to allow the wafers W to freely rotate. However, instead of the templates 112, an adhesive agent or wax may be used to bond and fix the wafers W to the lower faces of the carrier plates 110.
Since chips generated during polishing operations and slurry abrasive grains remain on the polishing pad, the polishing pad deteriorate, and the wafer polishing efficiency drops rapidly, as the wafer polishing operations continue. More specifically, since the surface of the polishing pad becomes too smooth, the slurry retention rate (slurry remaining rate) becomes lower. As a result, the slurry does not spread evenly on the polishing pad, and this phenomenon causes variations of wafer surface polishing conditions and a decrease in wafer polishing removal efficiently.
To counter this problem, seasoning is performed to recover the slurry retention, where the smoothened surface of the polishing pad is put into almost an initial state. A semiconductor polishing device having a center roller at the center of a polishing pad is now described as an example. As illustrated in FIG. 11-1, a semiconductor polishing device 200 has an urethane-foam polishing pad 206 bonded onto a round platen 204 that rotates coaxially with a center roller 202. The platen 204 rotates in the opposite direction (counterclockwise) from the rotation direction of the center roller 202 that rotates clockwise. Silicon wafers 210 having its polished surface facing downward are bonded to the lower face of a polishing plate 208 with wax. The polishing plate 208 has a smaller diameter than the radiuses of the platen 204 and the polishing pad 206. The side face of the polishing plate 208 is brought into contact with the side face of the center roller 202. A weight portion for facilitating the polishing of the silicon wafers 210 is placed on the polishing plate 208, or weight is applied onto the upper face of the polishing plate 208, so as to press the silicon wafers 210 against the polishing pad 206. While slurry (not shown) is supplied onto the polishing pad 206, the polishing pad 206 is rotated, and the polishing plate 208 is rotated counterclockwise by the friction caused by the rotation of the center roller 202. The surfaces of the silicon wafers 210 are polished by the friction caused between the silicon wafers 210 and the polishing pad 206 (or the abrasive grains in the slurry) by the rotation of the polishing pad 206 and the polishing plate 208. A ring-like conditioner (also called a dresser) 212 having an electrodeposited diamond grindstone is attached to a rotating member 214, and, like the polishing plate 208, the rotating member 214 is placed on the polishing pad 206. The rotating member 214 is rotated counterclockwise, to perform toothing on the surface of the polishing pad 206. In this manner, seasoning is performed. The seasoning is performed not only in the rough polishing process, but also in the CMP (Chemical Mechanical Polishing) process at a later stage (see JP-A No. 2002-208575 and 2003-151934).
However, if the seasoning with the use of the conditioner is repeated, the inner circumferential region 206a and the outer circumferential region 206c of the polishing pad 206 are selectively abraded. FIG. 11-2 illustrates this phenomenon more specifically. Where the abscissa axis indicating the moving radial direction of the polishing pad 206, and the ordinate axis indicates the abrasion depth in the polishing pad 206, deeper abrasion is observed in the inner circumferential region 206a and the outer circumferential region 206c, and the curve in FIG. 11-2 becomes a convex curve.
When a silicon wafer is polished with the use of a polishing pad having a shape represented by such a convex curve, the flatness of the surface of each silicon wafer becomes poorer, and more polishing is performed on the side of the silicon wafer positioned in the inner circumferential region of the polishing pad than on the side of the silicon wafer positioned in the outer circumferential region of the polishing pad. This phenomenon is called “inner abrasion”. This inner abrasion can be eliminated by lowering the rotation speed of the polishing pad. However, when the rotation speed becomes lower, the polishing efficiency also becomes lower. If the rotation speed of the polishing pad is lowered, “outer abrasion” might be observed, as opposed to the inner abrasion. However, depending on the surface condition of the polishing pad, a silicon wafer might have either inner abrasion or outer abrasion, even if the polishing pad is rotated at a fixed rotation speed. Therefore, it is difficult to control the flatness of the polished surface of each silicon wafer by adjusting the rotation speed of the polishing pad.
To solve this problem, JP-A No. 2003-151934 discloses a structure that controls the position of the polishing-pad seasoning conditioner on the polishing pad, and maintains the flatness of the polishing pad. However, this structure requires a mechanism for controlling the conditioner placed on the polishing pad. As a result, the structure becomes complicated, and the costs become higher. As shown in FIG. 12, Japanese Patent Publication No. 3,159,928 discloses a structure in which a quatrefoil hole 306 is formed in the lower face 302 of a conditioner 300 that has diamond abrasive grains 304 dispersed on the lower face 302 in contact with a polishing pad. The conditioner 300 is designed to flatten the polishing pad. In this structure, the efficiency of the seasoning of the polishing pad is increased in the inner circumferential region of the conditioner 300, and the polishing pad is flattened. However, unlike a conventional ring-like conditioner, the conditioner 300 has a complicated structure, resulting in higher costs.